It doesn't have brake lights like some VMs and I'm quite interested in looking into fitting some kind of braking sensor as I think that's useful.
There are two approaches to this:
1) Detect the brake being applied: This is a solved problem for e-bikes (in order to tell the controller to cut motor power or apply regenerative braking), and usually takes the form of a magnet/reed switch arrangement that is built into (or retrofitted to) the brake lever. You can also get optical sensors that fit inline on the brake cable.
2) Detect the vehicle slowing down: Measure the speed of a wheel, either using a traditional reed-switch speed sensor, or by measuring the frequency of a hub dynamo's output. Electronics decide when the rate of deceleration is sufficient to engage the light. Some B&M dynamo rear lights have this as a feature (getting brighter as the bike slows down).
I built a brake light using method 2) on my hybrid some years ago. It works about as well as the B&M solutions. The problem is that they're pretty much useless in the real world. Mainly because nobody expects a bicycle to have a brake light, so don't interpret it as such (probably not an issue on a velomobile), but also because there's always going to be a point where you're pedalling hard at lowish cadence and the spurts of speed cause the light to flash. Additionally, if you're using a magnet based sensor, there's a worst-case lag of 1 wheel revolution before the light comes on (vs 1/28th of a wheel rotation with a dynamo).
Therefore I recommend directly sensing the operation of the brakes themselves. They'll behave the way people expect brake lights to.